Mass Transport in Plants

Question 1

What is the role of the xylem?

Answer 1

To transport water up the plant to the leaves

Question 2

What is the role of the phloem?

Answer 2

To transport sugars and organic substances from the leaves (source) to where they are needed e.g. shoots, flowers, roots

Question 3

What are the 2 types of tissue involved in mass transport in plants?

Answer 3

Xylem and phloem

Question 4

Describe the structure of xylem vessels.

Answer 4

They are very long and tube-like structures formed from dead cells (vessel elements) joined end to end, there are no end walls on these cells, making an uninterrupted tube that allows water to pass up through the middle easily

Question 5

What is the cohesion tension theory?

Answer 5

1. Water is lost through the stoma and evaporates from the leaves at the ‘top’ of the xylem by transpiration.
2. Water pressure decreases.
3. Creating tension, water molecules are pulled up because water molecules are cohesive so when some water molecules are pulled up into the leaf others follow.
4. The water and cellulose cell wall of the xylem form hydrogen bonds - adhesion and pulls the walls together.
5. This means that the whole column of water in the xylem from the leaves down to the roots moves upwards.
6. Nitrates enter the root hair cell through active transport.
7. This causes the water potential to become more negative so then water moves in by osmosis.

Question 6

Water moves up a plant against what?

Answer 6

Gravity

Question 7

Water enters the stem of a plant through the?

Answer 7

Root hair cells

Question 8

What is transpiration?

Answer 8

The evaporation of water from a plant’s surface, especially the leaves

Question 9

Why does the plant need to open it’s stomata?

Answer 9

To let in CO2 so that it can produce glucose but also lets water out

Question 10

What happens in transpiration?

Answer 10

1. Water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf.
2. When the stomata open, it moves out of the leaf and down the concentration gradient (there is more water inside the leaf than in the air outside).

Question 11

What are the 4 main factors that affect transpiration rate?

Answer 11

Light, temperature, humidity and wind

Question 12

Explain how light affects the rate of transpiration?

Answer 12

The lighter it is the faster the transpiration rate because the stomata open when it gets light to let in CO2 for photosynthesis but when it is dark, the stomata are usually closed so there is little transpiration

Question 13

Explain how temperature affects the rate of transpiration?

Answer 13

The higher the temperature, the faster the transpiration rate, warmer water molecules have more energy so they evaporate from the cells inside the leaf faster which increases the concentration gradient between the inside and outside of the leaf, making water diffuse out of the leaf quicker

Question 14

Explain how humidity affects the rate of transpiration?

Answer 14

The lower the humidity, the quicker the transpiration rate, if the air around the plant is dry, the concentration gradient between the leaf and the air is increased which increases transpiration

Question 15

Explain how wind affects the rate of transpiration?

Answer 15

The windier it is, the faster the transpiration rate, lots of air movement blows away water molecules from around the stomata which increases the concentration gradient which increases the rate of transpiration

Question 16

Describe how to use a potometer.

Answer 16

1. Cut a shoot at a slant underwater.
2. Assemble the potometer in water and insert the shoot underwater.
3. Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water.
4. Check that the apparatus is water and air tight.
5. Dry the leaves and allow the shoot time to acclimatise and then shut the tap.
6. Remove the end of the capillary tube from the beaker of the water until one air bubble has formed, then put the end back into the water.
7. Record the starting position of the air bubble.
8. Start the stopwatch and record the distance moved by the bubble per unit time, the rate of air bubble movement is an estimate of the transpiration rate.

Question 17

What does a potometer measure?

Answer 17

The water uptake by a plant

Question 18

When using a potometer, why must the shoot be cut underwater and at a slant?

Answer 18

To prevent air from entering the xylem and to increase the surface area available for water uptake

Question 19

What is an air-bubble sometimes called?

Answer 19

The air-water meniscus

Question 20

What are the 2 important cell types in phloem?

Answer 20

Sieve tube elements and companion cells

Question 21

What are sieve tube elements?

Answer 21

They are living cells that form the tube for transporting solutes, they have no nucleus and few organelles

Question 22

What are companion cells?

Answer 22

They carry out living functions for sieve cells and there is a companion cell found in each sieve tube element

Question 23

Give an example of a living function that a companion cell carries out.

Answer 23

Providing the energy needed for the active site of solutes

Question 24

What is translocation?

Answer 24

The movement of solutes to where they are needed in a plant

Question 25

Translocation moves solutes from

Answer 25

source (leaves) to sinks (plant cells and roots)

Question 26

Describe the stages of the mass flow theory/translocation

Answer 26

1. Sugars made by photosynthesis (in leaf) are actively transported into phloem by companion cells
2. This lowers the water potential of sieve cells, therefore water enters from xylem via osmosis
3. This increases the pressure in phloem causing mass movement to the sink (root)
4. Sugars are used in respiration or are stored at sink
5. As sugars are removed from phloem, W.P. increases and water moves back into the xylem by osmosis

Question 27

What piece of evidence supports the downward flow of sugar in the mass flow theory?

Answer 27

Ringing experiments
- Cylinders of outer bark are removed from woody stems (removing the phloem)
- Material (solutes) accumulate on top of where bark is removed
- This shows material is moving downwards and has no where further to go

Question 28

What piece of evidence supports the pressure gradient of sugar in the mass flow theory?

Answer 28

Radioactive tracing
- Using a labelled aphid
- The stylet (mouthpiece) of aphid is inserted into the sieve tubes to feed on sap
- The aphid is anaesthetised and the stylet is cut off, leaving it in the plant
- As the sap is under pressure, it comes out of the stylet and can be collected for analysis

Question 29

What piece of evidence shows the direction of which sugars are moved inside the plant?

Answer 29

Radioactive labelling
- CO2 labelled with radioactive carbon is supplied to an illuminated plant leaf
- Radioactive carbon is fixed into the sugars from photosynthesis, translocation can be traced now
- The source and sink are places on photographic film in the dark for 24 hours
- The presence of radioactivity fogs up the film
- Shows that sugar is transported up and down the stem as radioactivity is found in both parts

Question 30

What are the objections against the mass flow theory?

Answer 30

• Existence of sieve plates which act as a barrier to flow
• Sucrose and amino acids have been shown to move at different rates and in different directions
  -Phloem has high O2 consumption suggesting presence of active transport

Question 31

Describe the cohesion-tension theory of water transport in the xylem. (5)

Answer 31

1 - Water lost from the leaf because of transpiration
2 - Lowers water potential of mesophyll/ leaf cells
3 - Water pulled up xylem, creating tension
4 - Water molecules ‘cohere’/ ‘stick’ together by hydrogen bonds
5 - Forming a continuous water column
6 - Adhesion of water (molecules) to the walls of the xylem

Question 32

Describe the mass flow hypothesis for the mechanism of translocation in plants. (4)

Answer 32

• In source/leaf sugars actively transported into phloem
• By companion cells
• Lowers water potential of sieve cell/tube and water enters by osmosis
• Increase in pressure causes mass movement (towards sink/root)
• Sugars used/converted in root for respiration for storage;

Question 33

Describe the processes involved in the transport of sugars in the plant stems. (5)

Answer 33

1. In source/leaf sugars actively transported into phloem
2. By companion cells
3. Lowers water potential of sieve cell/tube and water enters by osmosis
4. Increase in pressure causes mass movement (towards sink/root)
5. Sugars used/converted in root for respiration for storage

Question 34

Give 2 precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot (2)

Answer 34

• Seal joints / ensure airtight / ensure watertight
• Cut shoot under water
• Cut shoot at a slant
• Dry off leaves
• Insert into apparatus under water
• Ensure no air bubbles are present
• Shut tap
• Note where bubble is at start / move bubble to the start position

Question 35

A potometer measures the rate of water uptake rather than the rate of transpiration. Give 2 reasons why the potometer does not truly measure the rate of transpiration (2)

Answer 35

• Water used for support / turgidity;
• Water used in photosynthesis;
• Water produced in respiration;
• Apparatus not sealed / ‘leaks’;

Question 36

Use information from the graphs (don’t have), and your own knowledge of the cohesion-tension theory of water movement through a plant, to explain why the diameter of the trunk is smallest at midday (6)

Answer 36

• Diameter of trunk minimal at warmest/ brightest time of day
• Stomata open in light = more water loss
• Water evaporates more when warm/ more heat energy for water evaporation
• Hydrogen bonding between water molecules/ cohesion between water molecules
• Adhesion between water molecules and walls of xylem vessels
• Xylem pulled inwards by faster flow of water/ pulled in by tension

Question 37

Explain how xylem tissue is adapted for its function (4)

Answer 37

• Long cells/ tubes with no end walls
• Continuous water columns
• No cytoplasm/ no organelles
• To obstruct flow/ allows easier water flow
• Thickening/ lignin
• Support/ withstand tension/ keeps water in cells
• Pits in walls
• allow lateral movement/ get round blocked vessels

Question 38

What evidence is there against the mass flow theory (4)

Answer 38

• Not all solutes move at the same speed
• Sucrose moves at the same speed to all regions
• As hydrostatic pressure would suggest that they move faster to areas of low sucrose concentration
• Sieve plates seem to hinder mass flow (although they may be a structural adaptation to prevent bursting)

Question 39

How does transpiration lead to a negative pressure in the xylem? (4)

Answer 39

• Water evaporates from mesophyll cells into the air spaces beneath the stomata and then the air
• Water molecules are drawn up behind it as a result of cohesion (caused by hydrogen bonds)
• A column of water is pulled up the xylem by transpiration pull
• Transpiration pull puts the xylem under tension creating a negative pressure inside (hence, ‘cohesion tension theory’)